The mammalian gut harbors a vast association of microorganisms that are critical for optimizing host fitness and health. The interaction of bacteria with their hosts is complex, and ranges from pathogenic to beneficial. Beneficial properties of the intestinal flora include the extraction of nutrients from the host diet, priming ofthe host immune system, and development ofthe gut. The fruit fly, Drosophila melanogaster, is a powerful system for identifying and studying the effects of genes on behavior, aging, and disease. The effects of bacterial colonization ofthe Drosophila gut parallel many ofthe observations made in higher organisms, and the presence of bacteria can extend Drosophila lifespan. Preliminary results have identified a Drosophila mutant that is long-lived only in the presence of bacteria and exhibits distinct gut bacterial colonization. This mutant will be characterized and tools will be developed to study microorganisms harbored in the fly host. In Aim 1, the bacterial species necessary for conferring beneficial effects to fly longevity will be identified. The composition ofthe gut microbiota will be determined as a function of fly age and in response to different nutrient conditions, including under dietary restriction, a manipulation that extends lifespan for a broad range of organisms. Drosophila modulates its gut microbiota using a network of tools including antimicrobial peptides, reactive oxygen species, and acidic lysozymes. These systems will be studied and compared in mutant flies to identify the mechanisms and genes by which hosts affect gut bacterial colonization (Aim 2). To fully characterize host-microbe interactions, a genetic screen will be performed to identify bacterial genes that are critical for colonization of the fly host (Aim 3).